As the demand for smaller and smaller integrated circuit device features continues to increase, the need for improved illumination sources used for inspection of these ever-shrinking devices continues to grow. One such illumination source includes a laser-sustained plasma source. Laser-sustained plasma light sources are capable of producing high-power broadband light in the ultraviolet and visible portion of the electromagnetic spectrum. Laser-sustained light sources operate by focusing laser radiation into a gas volume in order to excite the gas, such as argon or xenon, into a plasma state, which is capable of emitting broadband light. This effect is typically referred to as “pumping” the plasma. The pump laser may include a continuous wave (CW) laser, a modulated laser source, or a pulsed laser source. In a general sense, laser-sustained plasma light sources display temperatures higher than those found in competing technologies, such as electrical discharge-sustained light sources. In turn, the higher temperatures achieved utilizing a laser-sustained plasma light source leads to a brighter light source and emitted light with shorter wavelengths.
For example, the brightness of a plasma sustained utilizing CW optical pumping is generally limited since standard laser-sustained techniques are generally insufficient. For example, merely increasing power of a pumping laser tends to merely cause the plasma to grow in size. This effect occurs because pumping light is absorbed in the cooler regions of the plasma, which tend to encompass a central hotter region of the plasma. In this sense, increasing pumping power tends to merely pump more power into cooler exterior regions of the plasma, while the core temperature of the plasma remains relatively unchanged.
Therefore, it would be desirable to provide a system and method for preferentially pumping a hotter region of the plasma, which corrects the deficiencies identified in the prior art.